1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording information on or reproducing recorded information from an optical information recording medium. In particular, the invention relates to an optical information recording/reproducing apparatus for recording information on or reproducing recorded information from a recording medium having at least two rows of information tracks in one groove or between grooves.
2. Related Background Art
In general, reproduction of information recorded in an optical information recording medium is carried out in such way that a minute spot is formed on a signal surface by a condensing lens and then a reflected light is received by a photoelectric conversion device. The demand for high densification of this optical information recording is increasing year by year.
As for a method of attaining this high densification, for example, there is known a method in which the high densification in a track direction is realized beyond restriction of optical resolution using means such as domain wall displacement detection (DWDD) means. For realizing the higher densification, it is necessary to narrow a track pitch to realize the densification in a radial direction. However, if a period of guide grooves is brought close to the resolution of an optical system, then a sufficient tracking signal can not be obtained. As well known, when a wavelength of a convergent light is assigned λ, and a numerical aperture of an objective lens is assigned NA, the resolution d of an optical system is determined on the basis of the following expression:d=λ/(2×NA)For example, since in an optical system as being used in DVD-RW, a wavelength λ of a light source is 635 nm, and a numerical aperture NA of an objective lens is 0.60, resolution d becomes 0.53 μm. If a track pitch of a medium is brought close to a value of 0.53 μm, a sufficient tracking signal can not be obtained. Thus, in DVD-RW, a track pitch is set to 0.74 μm and under this condition, a tracking signal is obtained.
For the purpose of relaxing such a restriction due to the resolution of the optical system, as described in Japanese Patent Application Laid-Open No. 2000-331383, for example, there is proposed an optical recording medium in which tracks are formed so that widths of adjacent grooves become different from each other. FIG. 16 shows a recording medium described in Japanese Patent Application Laid-Open No. 2000-331383. In the recording medium described in Japanese Patent Laid-Open No. 2000-331383, since a period structure is provided in which two tracks apparently correspond to one period, a tracking signal can be obtained even if a track pitch is narrowed down to a track pitch beyond a resolution limit.
However, in case of the recording medium described in Japanese Patent Laid-Open No. 2000-331383, it is necessary to form grooves having different widths on both sides of each track. Thus, in order that a tracking signal has sufficient modulation, it is necessary to increase a difference in width between the grooves. This is a cause for restricting the high densification.